What is described by the following?

This is a way to simulate shadows. We create this from the light's perspective by calculating the distances from the light source to the nearest lit fragment. We store these depths in this (a texture). During the final rendering, we compare the distance from the fragment to the light source with the distance in this. If the depth in this is less than the distance from the fragment to the source the fragment is in shadow (from this source). Otherwise, we use the rendered colour. What is this?